Hepatitis A virus (HAV) is a picornavirus with a .about.7.5 kb positive strand RNA genome and is the sole member of the Hepatovirus genus (Francki, R. I. B., et al. (1991) Classification and Nomenclature of Viruses. (Arch. Virol./Suppl. 2). Springer, Vienna). The clinical manifestations of HAV infection in humans can vary greatly, ranging from asymptomatic infection, commonly seen in young children, to fulminant hepatitis, which in some cases can result in death (Ross, B. C., et al. (1991) Adv. Virus Res., 39:209-253).
In attempting to prevent hepatitis A, three general strategies are possible: 1) increasing hygiene standards; 2) passive immunization of those known to be exposed to HAV with normal human immune globulin; and 3) the development of HAV vaccines. However, because sanitation levels in underdeveloped countries remain low and passive immunization offers little hope for control of endemic hepatitis A since most cases of hepatitis A occur in individuals who do not have a specific exposure history, considerable research efforts have been devoted to the development of either live or killed vaccines.
With respect to killed or inactivated vaccines, numerous laboratories have reported the development of inactivated HAV vaccines (see, for example, Binn, L. N. et al. (1986) J. Inf. Dis., 153:749; Provost, P. J. et al. (1986) J. Med. Virol., 19:23; Flehmig, B. et al. (1989) Lancet i:1039 and Andre, F. E. et al. (1990) Progress in Med. Virol., 37:72) and SmithKline Beecham and Merck have recently licensed and sold inactivated HAV vaccines containing different strains of HAV. However, the high cost of inactivated HAV vaccines makes their use in other than high-risk individuals unlikely. In addition, questions concerning the duration of immunity induced by inactivated HAV vaccines suggests that multiple doses may need to be administered to confer continued protection. Thus, for these reasons, the widespread use of live attenuated HAV vaccines in underdeveloped countries where hepatitis A is endemic may be more feasible and more efficacious than use of inactivated vaccines.
In attempting to develop a live attenuated vaccine, numerous investigators have selected attenuated hepatitis A viruses by passage of wild-type HAV strains in cell culture (see, for example, Provost et al. (1986) J. Med. Virol., 20:165-176; Karron, R. A. et al. (1988) J. Infect. Dis., 157:338-345). However, attenuation of HAV strains during adaptation to growth in cell culture has been observed to result in overattenuation such that the attenuated viruses, when administered as live vaccines,are no longer effective inducers of anti-HAV antibodies in vivo (Provost, P. J. et al. (1986) J. Med. Virol., 20: 165-175).
A potential alternative approach to the production of a candidate live attenuated vaccine strain which grows sufficiently well in a cell line to make vaccine production economically feasible and which is also infectious, immunogenic and avirulent in humans, is the use of recombinant DNA methodology to construct chimeric HAV genomes.